1. Field of the Invention
The present invention relates to a photoacoustic spectrometry device, used for example for analysing gases. It more particularly relates to a miniaturized photoacoustic spectrometer. This device is implemented from stacks of elementary components, capable of being implemented by substrate etching, metallic deposition and substrate assembly techniques, of the type used in microelectronics.
2. Discussion of the Background
The principle of photoacoustic spectroscopy for analysing gases has been discussed in the article by J. Christensen entitled xe2x80x9cThe Brxc3xcel Kjaer Photoacoustic Transducer System and its Physical Propertiesxe2x80x9d. The device described in this document has:
an infrared hot source,
a mechanical xe2x80x9cchopperxe2x80x9d, which modulates the intensity of the source,
an interferential optical filter,
a cylindrical trough,
two matched microphones, the sum of the signals originating from these microphones making it possible to double the photoacoustic signal and nullify the noise due to external vibrations.
In this device, the source is at a distance from the trough in order to avoid any heating of the gas. To that end, the use of an ellipsoidal mirror coupled to the source makes it possible to achieve suitable collimation of the light beam.
The document WO-96/24831 describes a photoacoustic detector having a chamber for receiving a gas to be measured, an infrared light beam capable of passing through this chamber, and a pressure sensor capable of measuring the pressure variations in the chamber, which are induced by an infrared beam. The chamber is formed by the assembly of two semiconductor elements, for example silicon or quartz elements, implemented in planar technology. The pressure variations are detected by means of a membrane.
Implementation of a photoacoustic spectrometer, of miniature size, allowing integration of all the elements (radiation source, filter, trough, microphone) in a compact manner, is not known.
An object of the invention is a photoacoustic spectrometer having an infrared source which can be modulated electrically, an interferential optical filter, a microphone and a micro-trough, each of these elements being integrated on a semiconductor substrate or on at least one, or with the help of at least one semiconductor substrate, for example made of silicon.
An object of the invention is therefore a photoacoustic microspectrometer, obtained by assembly or sealing of four elements integrated on a semiconductor: an infrared source which can be modulated electrically, an interferential optical filter, a micro-trough, and a microphone.
Each of the elements composing the spectrometer according to the invention can be integrated on, or implemented with the help of, one or two semiconductor substrates.
According to a first particular embodiment, an object of the invention is a photoacoustic spectrometer having:
an infrared source implemented in a first semiconductor substrate,
a filter implemented with the help of a second semiconductor substrate,
a micro-trough formed in a third semiconductor substrate,
a microphone implemented with the help of the third semiconductor substrate and a fourth semiconductor substrate.
According to a second particular embodiment, an object of the invention is a photoacoustic spectrometer, having:
an infrared source implemented in a first semiconductor substrate,
a Fabry-Pxc3xa9rot interferential filter formed with the help of a second and a third semiconductor substrates,
a micro-trough Implemented partially in the third semiconductor substrate and partially in a fourth semiconductor substrate,
a microphone implemented with the help of the fourth and a fifth semiconductor substrates.
According to a third particular embodiment, an object of the invention is a photoacoustic spectrometer, having:
an infrared source, implemented in a first semiconductor substrate,
a Fabry-Pxc3xa9rot interferential filter formed with the help of the first and a second semiconductor substrates,
a micro-trough implemented in the second semiconductor substrate,
a microphone implemented on the surface of a third semiconductor substrate.
According to a fourth particular embodiment, an object of the invention is a photoacoustic spectrometer, having:
an infrared source implemented in a first semiconductor substrate,
a Fabry-Pxc3xa9rot interferential filter implemented with the help of the first and a second semiconductor substrates,
a microphone and a micro-trough, implemented in the second semiconductor substrate.
In the device according to the invention, the mechanical chopping of the beam can be replaced by direct electrical modulation of the injection current in the infrared source.
The source can have a metallic grid, or a metallic filament, supported by a membrane above a cavity etched in a semiconductor substrate. This grid, or this filament, is for example made of silicon nitride, or platinum, or tantalum, or titanium, or tungsten, or molybdenum, or chromium, or nickel, or one of their alloys, or TiN.
Preferably, the source is placed in a cavity. Putting this cavity under vacuum moreover makes it possible to avoid heating problems due to the gaseous medium, which can be critical in a miniature device.
The interferential filter can be a filtering substrate.
This can also be a Fabry-Pxc3xa9rot tunable filter.
It can then have a first, fixed, mirror and a second, movable, mirror, these mirrors delimiting, at rest, a resonant cavity of length d, first and second control electrodes being associated respectively with these first and second mirrors, the application of an electrical voltage between the control electrodes allowing implementation of a displacement of the movable mirror with respect to the fixed mirror, and therefore modifying the length d of the resonant cavity.
According to another embodiment, the Fabry-Pxc3xa9rot tunable filter has:
a first mirror, with which a floating electrode is associated,
a second mirror, with which a first and a second control electrode are associated, one out of the first and second mirrors being fixed while the other is movable,
a resonant cavity, of length d, delimited by the first and second mirrors, the application of an electrical voltage between the two control electrodes bringing about a displacement of the movable mirror with respect to the fixed mirror and therefore modifying the length d of the resonant cavity.
In this embodiment, the electrode associated with one of the mirrors is a floating electrode, and no contact connection is to be implemented on the side of this mirror. There is therefore, in this system, only a single level of contact to be made, corresponding to the control electrodes. The device is therefore easier to implement, since a contact connection on both levels of mirror is difficult and requires a local stack of highly doped layers.
The fixed and movable mirrors can be implemented by stacking of multilayers at xcex/4, on the surface of semiconductor substrates.
The movable mirror can be implemented by a membrane situated above a cavity implemented in a semiconductor substrate.
The floating electrode and the corresponding mirror can be implemented on the surface of one of the semiconductor substrates.
The control electrodes and the corresponding mirror can be implemented on the surface of another of the semiconductor substrates.
For example, the movable mirror can be composed of a membrane etched in the second semiconductor substrate.
As for the control electrodes, they can be formed on either side of a reflective area of the mirror with which they are associated. In other words, this mirror has a reflective central area, and lateral areas on which the control electrodes are formed.
This reflective central area can have a circular form. This circular form, delimited by the control electrodes, allows, if the corresponding mirror is movable, a perfectly plane displacement of the movable reflective area, since the electrostatic attraction takes place only at the periphery of this area, which makes it possible to have a diaphragmed filter output.
The control electrodes can be implemented in a metallic deposit. Furthermore, electrical contacts can be made directly on the control electrodes, on the surface of the substrate on which they are formed.
Preferably, the control electrodes also form an input diaphragm of the micro-trough.
According to another aspect, one of the walls of the micro-trough is constituted by the microphone membrane.
According to yet another aspect of the invention, the microphone can have a membrane, a first electrode associated with, or formed on, this membrane, and a second electrode, the excess pressures in the micro-trough being detected by variation in the capacitance of the air gap defined by the first and second electrodes.
According to yet another aspect, the microphone has a membrane end a first electrode associated with this membrane, both implemented on a semiconductor substrate, and a second electrode implemented on another semiconductor substrate.
When one of the walls of the micro-trough is constituted by the microphone membrane, the said membrane can be implemented in a coped semiconductor material, the microphone also having a counter electrode. The latter makes it possible to detect the vibrations of the membrane by measuring the variation in the capacitance formed by the membrane of doped semiconductor material and the counter electrode.
The microphone membrane can then be situated on the surface of a substrate, etched under the membrane.
When the tunable filter has a membrane, the latter can be used as both a filter membrane and a microphone membrane. In this case, control of the filter and detection of the pressure variations are carried out with the help of one and the same system. The same electrodes as those which control the filter can then be used to measure the excess pressure created in the micro-trough, that is to say that filter and microphone are as it were combined, and reduced to a single common membrane. In this case, and in order not to create any parasitic photoacoustic signal in the cavity due to absorption of the gas to be measured or of another gas present at the same time, the cavity is preferably placed under a neutral atmosphere, for example under argon. In principle, the excess pressures created in the trough are sufficiently small for the tuning of the filter to the wavelength not to be lost: however, and in order to limit any detuning, means for automatic control position-wise of the membrane can be provided.